The present invention relates to a digital FM modulator and, more particularly, to a digital FM modulator used for transmitting digital data including a television signal and an audio signal with an FM modulation wave.
In a conventional digital FM modulator, as described in Japanese Patent Laid-Open No. 62-266903, a modulation index of a digitized modulation signal is designated by external multiplication data, and a center frequency is set by addition data. FIG. 2 shows an arrangement of the conventional digital FM modulator. In FIG. 2, a modulation signal input to a modulation signal input terminal 11 is digital/analog-converted by an A/D converter 1 and input to one terminal of a multiplier 2. Multiplication data 13 is input to the other input terminal of the multiplier 2 and multiplied by an output from the A/D converter 1, and the data is output from the multiplier 2 and input to one input terminal of an adder 3. Addition data 14 is input to the other input terminal of the adder 3 and added to an output from the multiplier 2, and the data is output from the adder 3 and input to a direct digital synthesizer (to be referred to as a DDS hereinafter) 4. The DDS 4 integrates data input to the DDS 4 every output period of a reference oscillator 5. The integrated data is used as a phase amount to obtain a sine wave from an output terminal 12. FIG. 3 shows the DDS 4. The DDS 4 includes an accumulator 40 and a ROM 43. The accumulator 40 includes an adder 41 using an output from the adder 3 as an input and a delay circuit 42 for delaying an output from the adder 41 to be fed back to the adder 41. The ROM 43 uses an output from the adder 41 as an address input. The adder 41 adds the output from the adder 3 to an output from the delay circuit 42 delayed for one period every sampling period and outputs the resultant value to the ROM 43 as an address signal. For example, when an output from the adder 3 is set to be N, a value obtained by accumulating N to the output data from the adder 3 is output as an f signal every sampling period. The ROM 43 stores Sine wave data at consecutive addresses starting from address 0 and is accessed in response to the address signal of the f signal output from the accumulator 40 so as to output the Sine wave data having an output frequency F. The DDS 4 is described in the above prior art. In this manner, the DDS 4 is operated with reference to the output frequency of the reference oscillator 5, thereby obtaining a frequency proportional to the content of the output data from the adder 3. Therefore, an FM modulation wave can be obtained from the output terminal 12, and the maximum frequency deviation and center frequency of the FM modulation wave are set by the multiplication data 13 and the addition data 14, respectively.
In the above conventional digital FM modulator, when the multiplication data is changed to change the maximum frequency deviation, not only the maximum frequency deviation but also the center frequency are changed. For this reason, the addition data must be set again.